The securement of concrete reinforcing tendons by convergently movable wedge assemblies is well known in the concrete prestressing art. Most such concrete anchorages employ a wedge assembly having a continuously tapered frusto-conical exterior surface which mates with a similarly formed frusto-conical bore in an anchor plate or housing. In addition to the conventional tapered wedge assembly, wedge assemblies have been devised in which the exterior surface of the wedge assembly includes a spiral array of adjacent convergently tapered camming areas which mate with a similarly formed bore in an anchor plate or housing. Such an anchorage is disclosed in U.S. Pat. No. 2,930,642.
It was subsequently discovered that anchorages of the type described in U.S. Pat. No. 2,930,642 could be extremely advantageously employed in a concrete prestressing method particularly well suited to the prestressing of large concrete tanks. This subsequent method is described in detail in U.S. Pat. No. 3,518,748. As is described in U.S. Pat. No. 3,518,748, one of the standard techniques which has been previously employed in post-tensioning concrete tanks was the use of threaded turnbuckles. The principal disadvantage found through the use of turnbuckles was that they depended upon threading the ends of the rods or tendons on which they were mounted, thus reducing the tensile strength of the tendons to undesirably low levels. Additionally, the turnbuckles which had been employed were formed of a material having approximately the same hardness as the tendons themselves. Thus, when turnbuckles were employed, the turnbuckle would freeze up as a result of galling of either the turnbuckle or tendon under the friction forces generated long before an axial load or tension force which was adequate could be generated by the turnbuckles in the tendons. The solution set forth in U.S. Pat. No. 3,518,748 was to couple the tendons together by a coupler having convergently movable wedges, tension the tendons by jacking them toward each other, relaxing the tendons so that the wedges would pick up most of the tension load, re-tensioning the tendons with the jacking means, and rotating the coupler to take up the slack in the coupler caused by re-tensioning. After the slack was removed, the jacking forces can be relaxed and the coupler will take up the full tension force on the tendon.
While the approach of U.S. Pat. No. 3,518,748 has been employed extensively, there are certain applications in which it may have disadvantages, and in some instances the tensioning, relaxing and re-tensioning approach has resulted in some instances of the anchorage being unable to hold the re-tensioned load, which has required undesirable replacement of wedge assemblies.
While the turnbuckle approach to concrete tank prestressing has the disadvantages of decreasing the overall strength of the reinforcing tendons by threading the same and freezing up of the turnbuckle under friction forces and galling at undesirably low tension levels, turnbuckles have the advantages of not requiring jacks, tensioning tongs or other specialized equipment. Thus, conventional and readily available tools can be used to rotate the turnbuckle elements and induce tension forces. Therefore, if the inherent reduction in tendon strength and reduction in the forces which can be induced by the turnbuckle can be overcome, the use of turnbuckle systems for post-tensioning concrete tanks would again become advantageous.